• Clinical science

Atrioventricular block (Heart block)

Abstract

Atrioventricular block (AV block) is characterized by an interrupted or delayed conduction between the atria and the ventricles. AV blocks are divided into three different degrees depending on the extent of the delay or interruption. First-degree blocks are identifiable on ECG by a prolonged PR interval. Most patients with first-degree block are asymptomatic, and the condition is usually an incidental finding. Second-degree AV blocks are further divided into two different subtypes. Mobitz type I, or Wenckebach, blocks exhibit a progressive prolongation of the PR interval that culminates in a non-conducted P wave (“dropped beat”). Most patients with Mobitz type I blocks are asymptomatic. The Mobitz type II block generates dropped QRS complexes at regular intervals (e.g. 2:1, 3:1, or 3:2), often leading to bradycardia. Symptoms of patients with Mobitz type II block range from fatigue to dyspnea, chest pain, and/or syncope. This fixed second-degree block frequently progresses to a third-degree block. A third-degree AV block involves total interruption of the electrical impulse between the atria and ventricles. The complete absence of conduction results in a ventricular escape mechanism, which may be dangerously slow and result in life-threatening bradycardia or Stokes-Adams attacks. Therefore, a third-degree AV block is an absolute indication for pacemaker placement.

Etiology

References:[1][2]

First-degree AV block

Definition

Characteristics

  • May be found in healthy individuals, e.g., in athletes with vagal tone
  • Usually asymptomatic
  • Often discovered incidentally on ECG

Treatment

  • Clinical assessment for underlying diseases (e.g., structural heart diseases, electrolyte imbalances)
  • Usually no specific treatment necessary
  • Follow-ups to evaluate progression of the disease
  • Pacemaker
    • Indication:

References:[3][4][5]

Second-degree AV block

Mobitz type I/Wenckebach

Definition

  • Progressive lengthening of the PR interval until a beat is dropped; regular atrial impulse does not reach the ventricles (a normal P wave is not followed by a QRS-complex)
  • Rate of SA node > heart rate; mostly regular rhythm separated by short pauses, which may lead to bradycardia

Symptoms/clinical findings

  • Usually asymptomatic
  • May present with symptoms of reduced cardiac output, resulting in hypoperfusion (e.g., dizziness, syncope; ) and bradycardia
  • Irregular pulse

Treatment

  • Asymptomatic patients
    • Clinical assessment for underlying diseases (e.g., structural heart diseases, electrolyte imbalances)
    • Usually no specific treatment necessary
    • Follow-ups (ECG and cardiac monitoring; ) to evaluate progression of the disease
  • Symptomatic patients
    • Hemodynamically stable
      • Monitoring with transcutaneous pacing pads
      • If symptoms not reversible; → placement of a permanent pacemaker
    • Hemodynamically unstable

Mobitz type II

Definition

  • Single or intermittent non-conducted P waves without QRS complexes
  • The PR interval remains constant.
  • The conduction of atrial impulses to the ventricles follows regular patterns:
    • 2:1 block: regular AV block that inhibits conduction of every other atrial depolarization (P wave) to the ventricles (heart rate = ½ SA node rate)
    • 3:1 block: regular AV block with 3 atrial depolarizations but only 1 atrial impulse that reach the ventricles (heart rate = ⅓ SA node rate)
    • 3:2 block: regular AV block with 3 atrial depolarizations but only 2 atrial impulses that reach the ventricles (heart rate = ⅔ SA node rate)

Symptoms/clinical findings

Treatment

  • Hemodynamically stable patients:
    • Monitoring with transcutaneous pacing pads
    • Clinical assessment for underlying diseases (e.g., structural heart diseases, electrolyte imbalances)
    • If symptoms are not reversible; → placement of a permanent pacemaker (mostly DDD devices)
  • Hemodynamically unstable patients:

The second-degree AV block Mobitz type II may progress to a third-degree block and is an unstable condition that requires monitoring and treatment!
References:[1][4][6][7]

Third-degree AV block

Definition

  • Third-degree AV block is a complete block with no conduction between the atria and ventricles.
  • On ECG, P waves and QRS complexes have their own regular rhythm but bear no relationship to each other (AV dissociation).
  • A ventricular escape mechanism is generated by sites that are usually located near the AV node or near the bundle of His.
  • Sudden onset of a third-degree AV block results in asystole, which lasts until the ventricular escape mechanism takes over. This asystole may lead to Stokes-Adams attacks.

Symptoms/clinical findings

Symptoms depend on:

  • Rate of ventricular escape mechanism
    • Bradycardia (< 40 bpm) with cerebral hypoperfusion (fatigue, irritability, apathy, dizziness, syncope, cognitive impairment), heart failure, dyspnea
  • Length of asystole

Treatment

  • Hemodynamically stable patients:
    • Monitoring with transcutaneous pacing pads
    • Clinical assessment for underlying diseases (e.g., structural heart diseases, electrolyte imbalances)
    • No reversible causes → placement of a permanent pacemaker
  • Hemodynamically unstable patients:

References:[8][9]

Differential diagnoses

  • Left posterior fascicular block (LPFB)
    • Block of the posterior fascicle of the left Tawara branch
    • ECG findings: (extreme) right axis deviation, rS pattern in leads I and aVL, qR pattern in leads III and aVF, QRS duration < 120 ms
    • Diagnosis LPFB via ECG is difficult, and requires that increased right ventricular volume load be ruled out as a cause of right axis deviation.
  • Left anterior fascicular block (LAFB)
    • Malfunction of the anterior fascicle of the left Tawara branch
    • Axis deviation to the left: extreme left axis deviation and persistent S wave in lead V6
  • Left bundle branch block
  • Right bundle branch block
  • SA nodal exit block
    • Definition: impaired impulse conduction from the SA node to atrial tissue
    • Classification:
      • First-degree SA block: delayed exit of impulses from the SA node with normal conduction to the atria
      • Second-degree SA block: periodically delayed conduction that can manifest in twodistinct forms
        • Type I (Wenckebach type): Progressive shortening of the P-P intervals followed by a pause. The duration of the pause is shorter than the duration of the two previous P-P intervals combined
        • Type II (Mobitz type): occasionally interrupted pacemaker activity in a periodic pattern, e.g., 2:1 or 3:1.
      • Third-degree SA block: Conduction from the SA node to the atria is completely interrupted. It can lead to asystole if no ventricular escape mechanism develops (e.g., of junctional or ventricular origin).

The differential diagnoses listed here are not exhaustive.